Twinning is a fundamental mechanism behind the simultaneous increase of strength and ductility in medium- and high-entropy alloys, but its operation is not yet well understood, which limits their exploitation. Since many high-entropy alloys showing outstanding mechanical properties are actually thermodynamically unstable at ambient and cryogenic conditions, the observed twinning challenges the existing phenomenological and theoretical plasticity models. Here, we adopt a transparent approach based on effective energy barriers in combination with first-principle calculations to shed light on the origin of twinning in high-entropy alloys. We demonstrate that twinning can be the primary deformation mode in metastable face-centered cubic alloys with a fraction that surpasses the previously established upper limit. The present advance in plasticity of metals opens opportunities for tailoring the mechanical response in engineering materials by optimizing metastable twinning in high-entropy alloys.

Significance: The ultimate goal of materials science can be reached only from a thorough understanding of the underlying physics of the materials properties. Accordingly, the present global need for high-technology metallic materials strongly depends on the development of robust theoretical understandings that can help to elucidate and optimize their performance. Although plastic deformation is a common denominator in many of these problems, no reliable microscopic theory of plasticity has been presented to date. We demonstrate that fundamental concepts in atomic theory provide a unified solution to plasticity and invite multidisciplinary contributions to deepen and extend the scope to a number of engineering applications. The present work will form a solid basis for many scientific and engineering disciplines.